Packaging containers for, for example, beverages and the like are often manufactured from a web or sheet-shaped packaging material which includes at least one surface of thermoplastic, i.e. heat-sealable or fusible material. The thermoplastic material makes it possible to seal the packaging material to itself or to other parts of the package, and imparts to the packaging material good liquid tightness. In the production of packaging containers from sheet or web-shaped packaging material, the material is often fused or sealed to itself with the aid of sealing jaws which heat and compress the material parts to one another so that the thermoplastic layers fuse together and form a liquid tight seal. Of course, the design and appearance of the sealing jaws is adapted to the particular type of material which is to be sealed, but a feature common to all prior art sealing jaws is that they display some form of thermal emission or thermal generation device. Thus, sealing jaws may, for example at their work surface, display a wire or a strip of electric resistor material which, when it is connected to a current source, is heated to a temperature which makes it possible to heat-seal thermoplastic material. If the packaging material which is to be sealed displays, in addition to a thermoplastic layer, a layer of metal such as aluminium foil, the heating can take place by means of induction heating, in which event a coil disposed in the sealing jaw induces a current in the aluminium foil so that this is heated and in turn conducts heat to the thermoplastic so that this can be sealed. A further possibility achieving for the desired temperature elevation in the thermoplastic layer is by utilizing ultrasonic sound, in which event the wave movement creates frictional heat which makes possible heat-sealing of the thermoplastic.
The above-mentioned methods all suffer from certain drawbacks. Induction heat (IH) for example functions only when the packaging material includes a layer of metallic material or when a layer of metal is otherwise provided within that region which is to be sealed. Ultrasonic heating requires an ultrasonic horn which is relatively large and can be difficult to accommodate at that point in a packaging machine where the sealing is to take place. The use of electric resistor material creates difficulties in design of the jaws, since the resistor material considerably changes shape on being heated and it hereby becomes difficult to connect with the work surface of the sealing jaw and an interjacent electrically insulating layer in such a manner that it is not subject to configurational change during lengthy periods of operation. This applies in particular in the use of electric resistor material for so-called impulse sealing, i.e. when the resistor material is heated during a relatively short period of time (100-600 ms) to a high temperature (120.degree.-450.degree. C.) in order to thereafter rapidly cool. This heating technique makes it necessary to employ a thermal strip of slight surface area, since otherwise the thermal inertia of the strip would be too great. In prior art sealing jaws of this type, it has proved difficult to wholly avoid movements between the jaw and the strip, with the result that the heating zone is changed so that sealing precision is lost. These movements are principally caused by the strip adhering to the surface layer of the packaging material and also showing a tendency to accompany the material when the sealing jaws are parted after sealing is completed. As a result, for example double-sided heating is prevented, i.e. the use of two mutually cooperating sealing jaws on either side of the laid-together packaging material which is to be sealed, since the precision and cooperation between the mutually registering strips is lost as a result of movements between the thermal strips and the sealing jaws. Attempts made hitherto to fix the thermal strip with the aid of welding, mechanical clamping or combinations thereof have resulted in clumsy designs and constructions with loose clamping devices which have hampered the design of the sealing jaw and reduced its versatility of use. In addition, the possibilities of various designs of the thermal strip and its active work surface facing towards the packaging material are considerably restricted.